Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead moves back and forth across the page shooting drops as it moves. To clean and protect the printhead, typically a service station is mounted within the printer chassis. For storage, or during non-printing periods, service stations usually include a capping system which humidically seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead.
During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting." Typically, the waste ink is collected in a stationary reservoir portion of the service station, which is often referred to as a "spittoon." After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide faster, more waterfast printing with darker blacks and more vivid colors, pigment based inks have been developed. These pigment based inks have a higher solids content than the earlier dye based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to use plain paper. Unfortunately, the combination of small nozzles and quick drying ink leaves the printheads susceptible to clogging, not only from dried ink and minute dust particles or paper fibers, but also from the solids within the new inks themselves.
Partially or completely blocked nozzles can lead to either missing or misdirected drops on the print media, either of which degrades the print quality. Thus, spitting to clear the nozzles becomes even more important when using pigment based inks, because the higher solids content contributes to the clogging problem more than the earlier dye based inks. Unfortunately, while stationary spittoons were suitable for the earlier dye based inks, they suffer a variety of drawbacks when used with newly developed pigment based inks.
For example, FIG. 8, is a vertical sectional view of a conventional prior art spittoon S which has been receiving waste ink of the newer variety for a period of time. The rapidly solidifying waste ink has gradually accumulated into a stalagmite I. The ink stalagmite I may eventually grow to contact the printhead H, which could interfere with printhead movement, print quality, and/or contribute to clogging the nozzles. Indeed, ink deposits along the sides of the spittoon often grow into stalagmites which can meet one another to form a bridge blocking the entrance to the spittoon. To avoid this phenomenon, conventional spittoons must be wide, often over 8 mm in width to handle these new pigment based inks. This extra width increases the overall printer width, resulting in additional cost being added to the printer, both in material and shipping costs.
This stalagmite problem is particularly acute for a polymer or a wax based ink, such as an ink based on carnauba wax, or a polyamide. In the past, inkjet printers using polyamide based inks have replaced the conventional spittoon of FIG. 8 with a sheet of flat plastic. The nozzles are periodically cleared by "spitting" the hot wax ink onto the plastic sheet. At regular intervals, an operator must remove this plastic sheet from the printer, flex the sheet over a trash can to remove the waste ink, and then replace the cleaned sheet in the printer. This cleaning step is particularly inconvenient for operators to perform on a regular basis, and is not suitable for the new pigment ink. In comparison to the wax or polymer based inks, these new inks leave a dirty, sticky residue, due to the high amount of solids used to improve the contrast and quality of the printed images. Thus, operator intervention to regularly clean a pigmented ink spittoon could lead to costly staining of clothing, carpeting, upholstery and the like.
In addition to increasing the solids content, mutually precipitating inks have been developed to enhance color contrast. For example, one type of color ink causes black ink to precipitate out of solution. This precipitation instantly fixes the black solids to the page, which prevents bleeding of the black solids into the color regions of the printed image. Unfortunately, if the mutually precipitating color and black inks are mixed together in a conventional spittoon, they do not flow toward a drain or absorbent material. Instead, once mixed, the black and color inks instantly coagulate into a gel, with some residual liquid being formed.
Thus, the mixed black and color inks have the drawbacks of hot-melt inks, which have an instant solid build-up, and the aqueous inks, which tend to run and "wick" (flow through capillary action) into undesirable locations. To resolve the mixing problem, two conventional stationary spittoons are required, one for the black ink and one for the color inks. As mentioned above, these conventional spittoons must be wide to avoid clogging from stalagmites growing inward from the spittoon sides. Moreover, using two spittoons further increases the overall width of the printer, which undesirably adds to the overall size of the inkjet printer, as well as its weight and material cost to build.
To maintain a high print quality in the hardcopy output, pens containing the new pigment based inks require new capping strategies. The pigment based inks have posed new challenges for efficiently capping the printheads. To maintain the desired ink characteristics, the area around the printhead nozzles must be kept clean and moist to prevent drying or decomposition of the ink during periods of printer inactivity. These principles are equally applicable to pens containing dye based inks.
In the past, a variety of different systems have been used to seal an inkjet printhead during periods of printer inactivity. These capping systems may be divided into three general categories based upon the direction of movement to engage the printheads, specifically, (1) linear caps, (2) vertical caps, and (3) rotary caps. The first group, linear caps, unfortunately require excessive carriage overtravel well beyond the print zone to seal the printheads. The mechanisms employed by these linear capping systems include an in-line four bar linkage mechanism, a ramp mounted sled, a four bar linkage including a spring mechanism, and combination ramp and spring mechanisms. Typically, these linear caps are pushed by the printhead in a direction parallel to the printhead scanning axis, and during this lateral motion, the caps are raised to seal the printhead nozzles.
Second, the vertical capping group of mechanisms move the caps upwardly to engage the printheads. One system uses a vertical rack and pinion mechanism, driven by a motor to move the caps upward to seal the printheads. Another vertical system uses a spring loaded vertical cam drive mechanism to cap the printheads.
The third capping system involves rotating the caps into position. One known rotary capping system rotates the caps about an axis which is perpendicular to the scanning axis of the printhead, and then cams the cap upward to engage the printhead. Another rotary system rotates a spring-biased lever to pivot the cap into a sealing position. This particular system gimbal-mounts the cap to the lever for limited angular tilting with respect to the printhead.
Unfortunately, each of these earlier capping systems has a variety of disadvantages. For example, many of them require extra carriage travel beyond the width required to mount the caps. This extra carriage travel results in a wider product with a large "footprint" (the work surface area occupied by the product). Some of these capping systems also have difficulty in sealing substantially irregular or nonplanar surfaces, such as those encountered when ink residue or other debris has accumulated on the printhead. These earlier systems also have difficulty in maintaining critical capping tolerances. Additionally, many of these earlier capping systems are sensitive to ink leakage from the pens, and accumulations of ink aerosol within the capping mechanism. The sticky aerosol and/or ink leakage build up may impede motion of critical components, leading to ineffective capping. Moreover, ink leakage from the capped pens often blocked or clogged vent ports within these earlier capping mechanisms.